1. Technical Field of the Invention
The present invention relates generally to a gas concentration measuring apparatus which may be used in measuring the concentration of a preselected component, such as oxygen, of exhaust emissions of automotive engines, and more particularly to such a gas concentration measuring apparatus designed to correct an output of a gas sensor for compensating for an output error arising from individual variability of the apparatus.
2. Background Art
Limiting current air-fuel (A/F) ratio sensors (also called A/F sensors or lambda sensors) are known which measure the concentration of oxygen (O2) contained in exhaust emissions of motor vehicle engines to determine an air-fuel ratio of a mixture supplied to the engine. A typical one of the A/F sensors includes a sensor element made up of a solid electrolyte body and a pair of electrodes affixed to the solid electrolyte body. The measurement of concentration of oxygen is achieved by applying the voltage to the solid electrolyte body through the electrodes to produce a flow of electrical current through the sensor element as a function of the concentration of oxygen and sampling the electrical current to determine the A/F ratio.
Usually, the A/F sensors or sensor control circuits therefore have individual variability in circuit characteristics, which will result in a decrease in accuracy of measuring the A/F ratio. In order to alleviate this problem, U.S. Pat. No. 5,925,088 (Japanese Patent No. 3257319) teaches a system designed to monitor an event that the A/F sensor is in a non-activated state, sample an actual output of the sensor control circuit in such an event, determine an error between the sampled output and a corresponding reference output, and correct an air-fuel ratio conversion map based on the determined error.
The above system, however, requires sampling an output of the sensor control circuit when the A/F sensor is in the non-activated state for compensating for the error of the output, which gives arise to the problem that a chance of finding the error of the output is limited only to a cold start phase of the A/F sensor. When the temperature of the sensor control circuit is increasing, it is, thus, impossible to find the error of the output precisely, which results in a decrease in accuracy of measuring the A/F ratio. Additionally, it is difficult to monitor the event that the A/F sensor is placed in the non-activated state accurately, which may lead to an error in correcting the air-fuel ratio conversion map.
Control systems for modern automotive gasoline engines are highly required to monitor an air-fuel ratio of a mixture around a stoichiometric air-fuel ratio with high accuracy. To this end, there has been proposed to expand a region around the stoichiometric air-fuel ratio for fine detection of the air-fuel ratio of the mixture. This approach has still left room for improvement of compensating for the error of the output.
U.S. Pat. No. 4,796,587 (Japanese Utility Model Second Publication No. 7-27391) teaches a system designed to select a correction factor used in compensating for an error of an output of an oxygen sensor in response to an identification signal indicative of an error inherent between an actual output of the sensor and a corresponding reference output. Usually, when an automotive engine is running at a high speed and a high load in response to an acceleration demand, it will result in an increased temperature of exhaust gas, which may cause parts of an exhaust system such as a catalytic converter, etc. to be overheated. In order to increase an engine output and avoid such overheating to protect the engine, typical engine control systems increase the amount of fuel supplied to the engine. This, however, causes an air-fuel mixture to be enriched excessively, thus resulting in an increase in fuel consumption. In order to address this problem, technologies are being developed to regulate the quantity of fuel to be injected into the engine using an output of the A/F sensor under rich feedback control.
A combination of the air-fuel ratio feedback control to control the air-fuel ratio around the stoichiometric one and in the lean region and the rich feedback control to control the air-fuel ratio in the rich region serve to ensure the stability in bringing the air-fuel ratio to around the stoichiometric one or in the lean region, but encounters a difficulty in attaining a target air-fuel ratio in the rich region for improving the fuel consumption. This is assumed to be due to a difference in output characteristics of the A/F sensor between when the air-fuel ratio is in the lean region and when it is in the rich region.